1. Field of the Invention
The invention relates to a device which detects a discharge caused by deterioration, an abnormality, or the like of an electric apparatus.
2. Description of the Related Art
A dielectric breakdown accident which is one of serious accidents of an electric power apparatus is often accompanied by a partial discharge as a forerunning phenomenon. When a partial discharge is detected, therefore, an abnormality in the insulation of an electric power apparatus can be detected at an early stage or in an insignificant state in which a dielectric breakdown accident will not be caused. In an electric power apparatus, the preventive maintenance technique is important. Consequently, it is requested to provide an electric power apparatus with a sensor for fault diagnosis.
In the case of a partial discharge due to insulation deterioration or the like, when such a discharge once occurs in an electric power apparatus, phenomena such as generation of an electromagnetic wave, variation of the electromagnetic field, generation of a discharge pulse current, generation of discharge noise, and chemical changes of insulating materials are produced. Conventionally, various methods of detecting a partial discharge which use such phenomena have been proposed.
FIGS. 9 and 10 are diagrams showing a prior art discharge detection method which is based on the detection of a discharge current and which is disclosed in Institute of Electrical Engineers of Japan, "State and Trend of Preventive Maintenance Technique for Transformer", Technical Report of Institute of Electrical Engineers of Japan (Part II), No. 344, August 1990 (Expert Committee on Preventive Maintenance of Transformer). In the figure, reference numeral 21 designates an object apparatus in which a discharge is to be detected, 22 designates a high-voltage capacitor, 23 designates a pulse transformer, 24 designates an amplifier, and 25 designates an observation apparatus such as an oscilloscope.
Next, the operation will be described. The high-voltage capacitor 22 is connected to the object apparatus 21, and grounded through the pulse transformer 23. The potential oscillation due to a discharge in the object apparatus 21 appears in the form of a charging/discharging pulse current of the high-voltage capacitor 22. This current is converted into a voltage by the pulse transformer 23. The voltage is amplified by the amplifier 24 and then observed on the oscilloscope 25 or the like.
FIG. 10 is a diagram showing another discharge detection method which is based on the detection of a discharge current. In the figure, reference numeral 21 designates an object apparatus in which a discharge is to be detected, 26 designates a Rogowski coil, 24 designates an amplifier, and 25 designates an observation apparatus such as an oscilloscope.
Next, the operation will be described. A pulse current due to a discharge or the like is taken out by the Rogowski coil 26, etc. which is electromagnetically coupled with a grounding conductor for the object apparatus 21. The pulse current is amplified by the amplifier 24 and then observed on the oscilloscope 25 or the like.
In both the methods, the frequency is in a wide band from hundreds kHz to about 1 MHZ. The circuit is connected to the apparatus via a conductor and captures a high-frequency signal generated over a wide range in a network of distributed constant circuits which are coupled with each other by spatial capacitances. Consequently, it is very difficult to distinguish an internal discharge to be observed from external noise. In a field, the S/N ratio is often so poor that the minimum detection level becomes high with the result that such a method cannot be practically used.
FIG. 11 is a diagram showing a prior art discharge detection method which is based on the detection of a discharge noise and which is disclosed in Institute of Electrical Engineers of Japan, "State and Trend of Preventive Maintenance Technique for Transformer", Technical Report of Institute of Electrical Engineers of Japan (Part II), No. 344, August 1990 (Expert Committee on Preventive Maintenance of Transformer). In the figure, reference numeral 27 designates an interior of an object apparatus which is filled with oil or the like, 28 designates a tank wall of the object apparatus, 29 designates a piezoelectric device, 30 designates a magnet plate, and 31 designates a cable. The components 29 to 31 constitute an ultrasonic microphone.
In the method, the ultrasonic microphone is attached to the tank wall 28 of the object apparatus and discharge noise is then detected. However, the sensitivity cannot be increased to a sufficient level. In a field, it is difficult to remove noise such as a sound produced by collision of rain drops or sand grains with the object apparatus.
In another prior art method, decomposition products are chemically analyzed. According to the method, a fluid insulator (oil or gas) in which organic insulators that are thermally decomposed by a discharge are dissolved is collected and then analyzed. An abnormality in the insulation is judged on the basis of the components of the collected gas. The judgment involves a prolonged time period, and requires expert knowledge of thermal decomposition characteristics of insulators and experience.
Prior art methods of detecting an electromagnetic wave generated by a discharge and components of the electromagnetic field are disclosed in Japanese Patent Unexamined Publication No. SHO 62-134574 "CORONA DISCHARGE DETECTOR", Japanese Patent Unexamined Publication No. HEI 2-297078 "ABNORMALITY DETECTOR FOR AN ELECTRIC APPARATUS", Japanese Patent Unexamined Publication No. HEI 3-81674 "PARTIAL DISCHARGE DETECTOR FOR AN ELECTRIC APPARATUS", and Japanese Patent Unexamined Publication HEI 3-239971 "CORONA DISCHARGE DETECTOR".
FIGS. 12A and 12B are diagrams showing the corona discharge detector disclosed in Japanese Patent Unexamined Publication No. SHO 62-134574 "CORONA DISCHARGE DETECTOR". In the figure, reference numeral 33 designates a conductor through which a high-voltage current flows and which is fixed to a metal duct 32 via support plates 34, and 35 designates an antenna which detects an electromagnetic wave generated by a corona discharge occurring in the duct and which is fixed to the duct 32. Only when the detection is to be conducted, the antenna 35 is connected to a control unit 37 via a connector 36.
Next, the operation will be described. When a corona discharge occurs, an electromagnetic wave is generated by the discharge. The antenna is previously disposed at various positions of the duct. In a patrol or the like, the portable control unit is connected to the antennas in sequence so that the detection is conducted. When an electromagnetic wave generated by a corona discharge is detected, it is judged that a corona discharge occurs.
In this system, the antennas are separated from the control unit, and hence the detect on cannot be conducted in real time. Consequently, the system has a drawback that a discharge cannot be detected unless an electromagnetic wave is generated when the control unit is connected to one of the antennas. In other words, the system is effective in the case where insulation deterioration advances to a level where a continued discharge is conducted, but may fail to detect a discharge in the case where insulation deterioration is in an initial stage and a discharge occurs only in a discrete manner.
FIGS. 13 and 14 show prior art examples disclosed in Japanese Patent Unexamined Publication No. HEI 2-297078 "ABNORMALITY DETECTOR FOR AN ELECTRIC APPARATUS" and Japanese Patent Unexamined Publication No. HEI 3-81674 "PARTIAL DISCHARGE DETECTOR FOR AN ELECTRIC APPARATUS". In the figures, reference numeral 38 designates a magnetic sensor in which a wire is wound around a magnetic core plural times in order to detect the magnetic field component of an electromagnetic wave generated by a partial discharge, 39 designates a sound receiving device which receives an ultrasonic wave generated by a partial discharge, 40 designates an amplifier-A, 41 designates an amplifier-B, 42 designates a discharge detection circuit, 37 designates a light receiving sensor for ultraviolet rays generated by a partial discharge, and 43 designates a display device.
Next, the operation will be described. Referring to FIG. 13, when a partial discharge occurs in an object apparatus 2, an electromagnetic wave and an ultrasonic wave are generated by the partial discharge. The electromagnetic wave is detected by the magnetic sensor 38. The detected electromagnetic wave is amplified by the amplifier-A 40 and then transmitted to the discharge detection circuit 42. The ultrasonic wave is detected by the sound receiving device 39. The detected ultrasonic wave is amplified by the amplifier-B 41 and then transmitted to the discharge detection circuit 42. When the discharge detection circuit receives inputs from both the magnetic sensor and the sound receiving device, the circuit judges that a discharge occurs, and the display device 43 is started to operate. When the discharge detection circuit receives an input from only one of the magnetic sensor and the sound receiving device, it is judged that there occurs a disturbance. In the apparatus shown in FIG. 14, the sound receiving device is replaced with the light receiving sensor for detecting ultraviolet rays generated by a partial discharge.
In this system, in order to distinguish a disturbance entering a magnetic sensor so as to improve the detection accuracy, the method in which an electromagnetic wave generated by a partial discharge is detected by the magnetic sensor is used together with the other method which uses the sound receiving device or the light receiving sensor. Consequently, the system has a drawback that the use of the plural sensors makes the system complicated in structure. The magnetic sensor has a structure in which a winding is formed on a magnetic core. Such a structure can detect an electromagnetic wave the frequency of which is several MHZ at the highest. Since electromagnetic waves of this frequency band are already used in many applications with leaving no space between frequencies, it is difficult to prevent a disturbance from entering the magnetic sensor, with the result that the system must be used conjointly with another system.
FIGS. 15A and 15B show a prior art example disclosed in Japanese Patent Unexamined Publication No. HEI 3-239971 "CORONA DISCHARGE DETECTOR". In the figure, reference numeral 45 designates a sensor unit, and 46 designates a group of micro-loop antennas which are different from each other in electromagnetic induction frequency and which cover the frequency range of 10 kHz to 1 GHz as a whole. Reference numeral 47 designates a coaxial cable, 48 designates an apparatus to be measured, 49 designates a comparison detector, 50 designates a dummy antenna for noise detection, 51 designates a CPU, 52 designates a RAM, 53 designates a ROM, 54 designates a CRT, and 55 designates a printer.
Next, the operation will be described. An electromagnetic wave is generated by a corona discharge occurring as a result of insulation deterioration of the apparatus 48 to be measured. The components of 10 kHz to 1 GHz of the electromagnetic wave are received by the antenna 46 and then supplied to the comparison detector 49 through the coaxial cable 47. Noise of the space is received by the dummy antenna 50 and then supplied to the comparison detector 49. The electromagnetic wave generated by the discharge, and the noise are compared with each other, and the comparison output is produced by the comparison detector. The comparison output is processed by the CPU 51 and then displayed on the CRT 54 and the printer 55.
In this system, since an electromagnetic wave generated by a discharge must be received in a wide band from 10 kHz to 1 GHz, a plurality of antennas are required. This frequency band is already used in many applications, and hence an external antenna for reference must be disposed in order to distinguish the electromagnetic wave from external noise.
Since the prior art methods of detecting a discharge are configured as described above, the methods are easily affected in a field by a disturbance. In order to correctly capture a discharge phenomenon, therefore, plural detection methods and a noise eliminating apparatus must be conjointly used.
In the prior art methods in which an electromagnetic wave generated by a discharge is detected, the object frequency is not higher than 1 GHz. An electromagnetic wave generated by a discharge has a wide frequency spectrum as shown in FIG. 16 which extends from a low frequency of several kHz to a microwave of ten-odd GHz, and has a characteristic in which its distribution is substantially proportional to 1/f (where f is a frequency). When a discharge is to be detected on the basis of an electromagnetic wave, therefore, the detection can be conducted efficiently in a configuration in which an electromagnetic wave of a low frequency is received. In the prior art, a configuration which utilizes a microwave is not used. The low-frequency region useful in the configuration in which an electromagnetic wave of a low frequency is received is in a range where electromagnetic waves are used in an overcrowded manner. Therefore, the configuration has a defect that it is easily affected by another electromagnetic wave, and hence must be used conjointly with a noise/disturbance removing apparatus.